Method using block copolymers for making a master mold with high bit-aspect-ratio for nanoimprinting patterned magnetic recording disks

ABSTRACT

The invention is a method for making a master mold to be used for nanoimprinting patterned-media magnetic recording disks. The method uses conventional optical or e-beam lithography to form a pattern of generally radial stripes on a substrate, with the stripes being grouped into annular zones or bands. A block copolymer material is deposited on the pattern, resulting in guided self-assembly of the block copolymer into its components to multiply the generally radial stripes into generally radial lines of alternating block copolymer components. The radial lines of one of the components are removed and the radial lines of the remaining component are used as an etch mask to etch the substrate. Conventional lithography is used to form concentric rings over the generally radial lines. After etching and resist removal, the master mold has pillars arranged in circular rings, with the rings grouped into annular bands.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to patterned-media magnetic recordingdisks, wherein each data bit is stored in a magnetically isolated dataisland on the disk, and more particularly to a method for making amaster mold to be used for nanoimprinting the patterned-media disks.

2. Description of the Related Art

Magnetic recording hard disk drives with patterned magnetic recordingmedia have been proposed to increase data density. In patterned media,the magnetic recording layer on the disk is patterned into smallisolated data islands arranged in concentric data tracks. To produce therequired magnetic isolation of the patterned data islands, the magneticmoment of spaces between the islands must be destroyed or substantiallyreduced to render these spaces essentially nonmagnetic. In one type ofpatterned media, the data islands are elevated regions or pillars thatextend above “trenches” and magnetic material covers both the pillarsand the trenches, with the magnetic material in the trenches beingrendered nonmagnetic, typically by “poisoning” with a material likesilicon (Si). Patterned-media disks may be longitudinal magneticrecording disks, wherein the magnetization directions are parallel to orin the plane of the recording layer, or perpendicular magnetic recordingdisks, wherein the magnetization directions are perpendicular to orout-of-the-plane of the recording layer.

One proposed method for fabricating patterned-media disks is bynanoimprinting with a template or mold, sometimes also called a“stamper”, that has a topographic surface pattern. In this method themagnetic recording disk substrate with a polymer film on its surface ispressed against the mold. The polymer film receives the reverse image ofthe mold pattern and then becomes a mask for subsequent etching of thedisk substrate to form the pillars on the disk. The magnetic layer andother layers needed for the magnetic recording disk are then depositedonto the etched disk substrate and the tops of the pillars to form thepatterned-media disk. The mold may be a master mold for directlyimprinting the disks. However, the more likely approach is to fabricatea master mold with a pattern of pillars corresponding to the pattern ofpillars desired for the disks and to use this master mold to fabricatereplica molds. The replica molds will thus have a pattern of holescorresponding to the pattern of pillars on the master mold. The replicamolds are then used to directly imprint the disks. Nanoimprinting ofpatterned media is described by Bandic et al., “Patterned magneticmedia: impact of nanoscale patterning on hard disk drives”, Solid StateTechnology S7+ Suppl. S, SEP 2006; and by Terris et al., “TOPICALREVIEW: Nanofabricated and self-assembled magnetic structures as datastorage media”, J. Phys. D: Appl. Phys. 38 (2005) R199-R222.

In patterned media, there are two opposing requirements relating to thebit-aspect-ratio (BAR) of the pattern or array of discrete data islandsarranged in concentric tracks. The BAR is the ratio of track spacing orpitch in the radial or cross-track direction to the island spacing orpitch in the circumferential or along-the-track direction, which is thesame as the ratio of linear island density in bits per inch (BPI) in thealong-the-track direction to the track density in tracks per inch (TPI)in the cross-track direction. The BAR is also equal to the ratio of theradial dimension of the bit cell to the circumferential dimension of thebit cell, where the data island is located within the bit cell. The bitcell includes not only the magnetic data island but also one-half of thenonmagnetic space between the data island and its immediately adjacentdata islands. The data islands have an island aspect ratio (IAR) orradial length to circumferential that is generally close to the BAR. Thefirst requirement is that to minimize the resolution requirement forfabricating the islands, it is preferable that the array of islands havea low BAR (about 1). The second requirement is that to allow for a widerwrite head pole, which is necessary for achieving a high write field toallow the use of high coercivity media for thermal stability, it ispreferable that the array of islands have a higher BAR (about 2 orgreater). Also, the transition from disk drives with conventionalcontinuous media to disk drives with patterned media is simplified ifthe BAR is high because in conventional disk drives the BAR is betweenabout 5 to 10. Other benefits of higher BAR include lower track density,which simplifies the head-positioning servo requirements, and a higherdata rate.

The making of the master template or mold is a difficult and challengingprocess. The use of electron beam (e-beam) lithography using a Gaussianbeam rotary-stage e-beam writer is viewed as a possible method to make amaster mold capable of nanoimprinting patterned-media disks with a BARof about 1 with a track pitch (island-to-island spacing in the radial orcross-track direction) of about 35 nm, and an island pitch(island-to-island spacing in the circumferential or along-the-trackdirection) of about 35 nm. If the data islands have a radial length andcircumferential width each of about 20 nm for an IAR of 1, then thesedimensions generally limit the areal bit density of patterned-mediadisks to about 500 Gbit/in². To achieve patterned-media disks with bothan ultra-high areal bit density (around 1 Terabits/in²) and a higherBAR, a track pitch of 50 nm and an island pitch of about 12.5 nm will berequired, which would result in a BAR of 4. However, a master moldcapable of nanoimprinting patterned-media disks with an island pitch of12.5 nm is not achievable with the resolution of e-beam lithography.

What is needed is a master mold and a method for making it that canresult in patterned-media magnetic recording disks with both therequired high areal bit density and higher BAR (about 2 or greater).

SUMMARY OF THE INVENTION

The invention is a method for making a master mold to be used fornanoimprinting patterned-media magnetic recording disks with a BARgreater than 1, preferably about 2 or greater. The method usesconventional optical or e-beam lithography to form a pattern ofgenerally radial stripes on a substrate, with the stripes being groupedinto annular zones or bands. A block copolymer material is deposited onthe pattern, resulting in guided self-assembly of the block copolymerinto its components to multiply the generally radial stripes intogenerally radial lines of alternating block copolymer components. Theradial lines of one of the components are removed and the radial linesof the remaining component are used as an etch mask to etch thesubstrate. Conventional lithography is used to form concentric ringsover the generally radial lines. After etching and resist removal, themaster mold has pillars arranged in circular rings, with the ringsgrouped into annular bands. The spacing of the concentric rings isselected so that following the etching process the master mold has anarray of pillars with the desired BAR, which is greater than 1,preferably about 2 or greater. The master mold may be used to directlynanoimprint the disks, but more likely is used to make replica moldswhich are then used to directly nanoimprint the disks.

The block copolymer may be a diblock copolymer of A and B componentshaving the structure (A-b-B), such aspolystyrene-block-polymethylmethacrylate (PS-b-PMMA). The ratio of themolecular weight of the A component to the molecular weight of the Bcomponent is selected so that the radial lines of the A component areformed either as cylinders in a matrix of the B component or asalternating lamellae separated by alternating lamellae of the Bcomponent. The two or more immiscible polymeric block componentsmicrophase separate into two or more different microdomains on ananometer scale and thereby form ordered patterns of isolated nano-sizedstructural units having a periodicity or bulk period (L₀) of therepeating A-B domain units. The block copolymer is selected to have L₀in the range of between about 8 nm and 25 nm, which corresponds to thecircumferential spacing of the A-component radial lines to be used asthe etch mask. However, the generally radial stripes used to guide theself-assembly of the block copolymer into its A and B components have acircumferential spacing of approximately nL₀, where n is an integergreater than or equal to 2.

For a fuller understanding of the nature and advantages of the presentinvention, reference should be made to the following detaileddescription taken together with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a top view of a disk drive with a patterned-media type ofmagnetic recording disk as described in the prior art.

FIG. 2 is a top view of an enlarged portion of a patterned-media type ofmagnetic recording disk showing the detailed arrangement of the dataislands in one of the bands on the surface of the disk substrate.

FIG. 3 is a side sectional view of one type of a patterned-media diskshowing the data islands as elevated, spaced-apart pillars that extendabove the disk substrate surface with trenches between the pillars.

FIG. 4 is a schematic view of a patterned-media disk showing a patternof radial lines in three annular bands, with each radial line meant torepresent data islands from all the concentric tracks in the band.

FIGS. 5A, 5B and 5C are views of a small portion of one annular band ofthe master mold at successive stages of the method of making the mastermold according to the present invention.

FIGS. 6A-6C are side sectional views, at various stages of oneembodiment of the method for making the master mold, taken through aplane generally perpendicular to the radial direction.

FIG. 6D is a top view of one stage of one embodiment of the method formaking the master mold.

FIGS. 6E-6G are side sectional views, at various stages of oneembodiment of the method for making the master mold, taken through aplane generally perpendicular to the radial direction.

FIG. 6H is a top view of one stage of one embodiment of the method formaking the master mold.

FIG. 6I is a side sectional view of one stage of one embodiment of themethod for making the master mold, taken through a plane generallyperpendicular to the radial direction.

FIGS. 6J-6L are top views at various stages of one embodiment of themethod for making the master mold.

FIGS. 6M-6Q are side sectional views, at various stages of analternative embodiment of the method shown in FIGS. 6C-6D, taken througha plane generally perpendicular to the radial direction.

FIG. 7A is a side sectional view at one stage of a variation of themethod depicted in FIGS. 6A-6L, taken through a plane generallyperpendicular to the radial direction.

FIGS. 7B-7E are top views at various stages of a variation of the methoddepicted in FIGS. 6A-6L.

FIGS. 8A-8C are side sectional views at various stages of anotherembodiment of the method for making the master mold, taken through aplane generally perpendicular to the radial direction.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a top view of a disk drive 100 with a patterned magneticrecording disk 10 as described in the prior art. The drive 100 has ahousing or base 112 that supports an actuator 130 and a drive motor forrotating the magnetic recording disk 10 about its center 13. Theactuator 130 may be a voice coil motor (VCM) rotary actuator that has arigid arm 134 and rotates about pivot 132 as shown by arrow 124. Ahead-suspension assembly includes a suspension 121 that has one endattached to the end of actuator arm 134 and a head carrier 122, such asan air-bearing slider, attached to the other end of suspension 121. Thesuspension 121 permits the head carrier 122 to be maintained very closeto the surface of disk 10. A magnetoresistive read head (not shown) andan inductive write head (not shown) are typically formed as anintegrated read/write head patterned on the trailing surface of the headcarrier 122, as is well known in the art.

The patterned magnetic recording disk 10 includes a disk substrate 11and discrete data islands 30 of magnetizable material on the substrate11. The data islands 30 function as discrete magnetic bits for thestorage of data and are arranged in radially-spaced circular tracks 118,with the tracks 118 being grouped into annular bands 119 a, 119 b, 119c. The grouping of the data tracks into annular bands permits bandedrecording, wherein the angular spacing of the data islands, and thus thedata rate, is different in each band. In FIG. 1, only a few islands 30and representative tracks 118 are shown in the inner band 119 a and theouter band 119 c. As the disk 10 rotates about its center 13 in thedirection of arrow 20, the movement of actuator 130 allows theread/write head on the trailing end of head carrier 122 to accessdifferent data tracks 118 on disk 10. Rotation of the actuator 130 aboutpivot 132 to cause the read/write head on the trailing end of headcarrier 122 to move from near the disk inside diameter (ID) to near thedisk outside diameter (OD) will result in the read/write head making anarcuate path across the disk 10.

FIG. 2 is a top view of an enlarged portion of disk 10 showing thedetailed arrangement of the data islands 30 in one of the bands on thesurface of disk substrate 11 according to the prior art. The islands 30are shown as being circularly shaped and thus have a BAR of 1. Theislands 30 contain magnetizable recording material and are arranged intracks spaced-apart in the radial or cross-track direction, as shown bytracks 118 a-118 e. The tracks are typically spaced apart by a nearlyfixed track pitch or spacing TS. Within each track 118 a-118 e, theislands 30 are roughly equally spaced apart by a nearly fixedalong-the-track island pitch or spacing IS, as shown by typical islands30 a, 30 b, where IS is the spacing between the centers of two adjacentislands in a track. The islands 30 are also arranged into generallyradial lines, as shown by radial lines 129 a, 129 b and 129 c thatextend from disk center 13 (FIG. 1). Because FIG. 2 shows only a verysmall portion of the disk substrate 11 with only a few of the dataislands, the pattern of islands 30 appears to be two sets ofperpendicular lines. However, tracks 118 a-118 e are concentric ringscentered about the center 13 of disk 10 and the lines 129 a, 129 b, 129c are not parallel lines, but radial lines extending from the center 13of disk 10. Thus the angular spacing between adjacent islands asmeasured from the center 13 of the disk for adjacent islands in lines129 a and 129 b in a radially inner track (like track 118 e) is the sameas the angular spacing for adjacent islands in lines 129 a and 129 b ina radially outer track (like track 118 a).

The generally radial lines (like lines 129 a, 129 b, 129 c) may beperfectly straight radial lines but are preferably arcs orarcuate-shaped radial lines that replicate the arcuate path of theread/write head on the rotary actuator. Such arcuate-shaped radial linesprovide a constant phase position of the data islands as the head sweepsacross the data tracks. There is a very small radial offset between theread head and the write head, so that the synchronization field used forwriting on a track is actually read from a different track. If theislands between the two tracks are in phase, which is the case if theradial lines are arcuate-shaped, then writing is greatly simplified.

Patterned-media disks like that shown in FIG. 2 may be longitudinalmagnetic recording disks, wherein the magnetization directions in themagnetizable recording material are parallel to or in the plane of therecording layer in the islands, or perpendicular magnetic recordingdisks, wherein the magnetization directions are perpendicular to orout-of-the-plane of the recording layer in the islands. To produce therequired magnetic isolation of the patterned data islands, the magneticmoment of the regions between the islands must be destroyed orsubstantially reduced to render these spaces essentially nonmagnetic.Patterned media may be fabricated by any of several known techniques. Inone type of patterned media, the data islands are elevated, spaced-apartpillars that extend above the disk substrate surface to define troughsor trenches on the substrate surface between the pillars. This type ofpatterned media is shown in the sectional view in FIG. 3. In this typeof patterned media the substrate 11 with a pre-etched pattern of pillars31 and trenches or regions between the pillars can be produced withrelatively low-cost, high volume nanoimprinting process using a mastertemplate or mold. The magnetic recording layer material is thendeposited over the entire surface of the pre-etched substrate to coverboth the ends of the pillars 31 and the trenches between the pillars 31,resulting in the data islands 30 of magnetic recording layer materialand trenches 32 of magnetic recording layer material. The trenches 32 ofrecording layer material may be spaced far enough from the read/writehead to not adversely affect reading or writing to the recording layermaterial in islands 30, or the trenches may be rendered nonmagnetic by“poisoning” with a material like Si. This type of patterned media isdescribed by Moritz et al., “Patterned Media Made From Pre-EtchedWafers: A Promising Route Toward Ultrahigh-Density Magnetic Recording”,IEEE Transactions on Magnetics, Vol. 38, No. 4, July 2002, pp.1731-1736.

FIG. 4 is a schematic view of patterned-media disk 10 showing a patternof radial lines in three annular bands 119 a-119 c. Each radial line ismeant to represent data islands from all the concentric tracks in theband. The circumferential density of the radial lines is similar in allthree bands, with the angular spacing of the lines being adjusted in thebands to have smaller angular spacing in the direction from the diskinside diameter (ID) to outside diameter (OD), so that thecircumferential density of the radial lines, and thus the “linear” oralong-the-track density of data islands, stays relatively constant overall the bands on the disk. In actuality, a typical disk is divided intoabout 20 annular bands, which allows the linear density to remainconstant to within a few percent across all bands. Within each band, theradial lines are subdivided (not shown) into very short radial segmentsor lengths arranged in concentric rings, with each ring being a datatrack and each radial segment or length being a discrete data island.Each annular band, like band 119 c, has a band ID and a band OD. Also,in actuality the generally radial lines are more typically generallyarcuate lines that replicate the path of the read/write head mounted onthe end of the rotary actuator.

The making of the master template or mold to achieve an ultrahighdensity patterned-media disk is a difficult and challenging process. Theuse of electron beam (e-beam) lithography using a Gaussian beamrotary-stage e-beam writer is viewed as a possible method to make themaster mold. However, to achieve patterned-media disks with both higherareal bit density (around 1 Tbit/in²) and a higher BAR, a track pitch ofabout 50 nm and an island pitch of about 12.5 nm will be required, whichwould result in a BAR of 4. A master mold capable of nanoimprintingpatterned-media disks with an island pitch of 12.5 nm is difficult tofabricate due to the limited resolution of e-beam lithography.

The present invention relates to a method for making a master mold thatis used in the nanoimprinting process to make patterned-media disks withan island pitch difficult to achieve with the resolution of e-beamlithography, thus enabling both higher areal bit density (1 Tbit/in² andhigher) and a high BAR (greater than 1). The master mold may be used todirectly nanoimprint the disks, but more likely is used to make replicamolds which are then used to directly nanoimprint the disks. The methoduses conventional or e-beam lithography to form a pattern of generallyradial stripes on a substrate, with the stripes being grouped intoannular zones or bands. A block copolymer material is deposited on thepattern, resulting in guided self-assembly of the block copolymer intoits components to multiply the generally radial stripes into generallyradial lines. The radial lines preferably have a higher circumferentialdensity than that of the radial stripes. Conventional lithography isthen used to form concentric rings over the generally radial lines.After etching and resist removal, the master mold has pillars arrangedin circular rings, with the rings grouped into annular bands. Thespacing of the concentric rings is selected so that following theetching process the master mold has an array of pillars with the desiredBAR, which is greater than 1, preferably about 2 or greater. Because theinvention allows the circumferential density of the master mold pillarsto be at least doubled from what could be achieved with just e-beamlithography, the subsequently nanoimprinted patterned-media disks canhave both a high BAR (greater than 1 and preferably about 2 or greater)and an ultra-high areal density.

A high-level representation of the method of the invention is shown inFIGS. 5A-5C, which show a small portion of one annular band of themaster mold with the radial or cross-track direction being vertical andthe circumferential or along-the-track direction being horizontal. InFIG. 5A, the first step is to create a pattern of generally radialstripes 204 on substrate 200 at a density achievable by conventionale-beam or other lithography. Next, in FIG. 5B, the circumferentialdensity of radial stripes 204 is multiplied by two as a result of guidedself-assembly of block copolymer material into its components, resultingin generally radial lines 212 representing one of the block copolymercomponents. The radial lines 212 are used as an etch mask to etch radiallines in the substrate and a second conventional e-beam or otherlithography step is performed to cut the radial lines of substratematerial into circumferential segments 213 of pillars 228. The pillars228 correspond to the data islands and the segments 213 correspond tothe data tracks on the disks that will be nanoimprinted. The pillars 228have a circumferential pitch difficult to achieve with the resolution ofe-beam lithography. The array of pillars 228 has a BAR greater than 1,preferably about 2 or greater.

Self-assembling block copolymers have been proposed for creatingperiodic nanometer (nm) scale features. A self-assembling blockcopolymer typically contains two or more different polymeric blockcomponents, for example components A and B, that are immiscible with oneanother. Under suitable conditions, the two or more immiscible polymericblock components separate into two or more different phases ormicrodomains on a nanometer scale and thereby form ordered patterns ofisolated nano-sized structural units. There are many types of blockcopolymers that can be used for forming the self-assembled periodicpatterns. If one of the components A or B is selectively removablewithout having to remove the other, then an orderly arranged structuralunits of the un-removed component can be formed. There are numerousreferences describing self-assembling block copolymers, including U.S.Pat. No. 7,347,953 B2; Kim et al., “Rapid Directed Self-Assembly ofLamellar Microdomains from a Block Copolymer Containing Hybrid”, Proc.of SPIE Vol. 6921, 692129, (2008); Kim et al., “Device-Oriented DirectedSelf-Assembly of Lamella Microdomains from a Block Copolymer ContainingHybrid”, Proc. of SPIE Vol. 6921, 69212B, (2008); and Kim et al.,“Self-Aligned, Self-Assembled Organosilicate Line Patterns of 20 nmHalf-Pitch from Block Copolymer Mediated Self-Assembly”, Proc. of SPIEVol. 6519, 65191H, (2007).

Specific examples of suitable block copolymers that can be used forforming the self-assembled periodic patterns include, but are notlimited to: poly(styrene-block-methyl methacrylate) (PS-b-PMMA),poly(ethylene oxide-block-isoprene) (PEO-b-PI), poly(ethyleneoxide-block-butadiene) (PEO-b-PBD), poly(ethylene oxide-block-styrene)(PEO-b-PS), poly(ethylene oxide-block-methylmethacrylate) (PEO-b-PMMA),poly(ethyleneoxide-block-ethylethylene) (PEO-b-PEE),poly(styrene-block-vinylpyridine) (PS-b-PVP),poly(styrene-block-isoprene) (PS-b-PI), poly(styrene-block-butadiene)(PS-b-PBD), poly(styrene-block-ferrocenyldimethylsilane) (PS-b-PFS),poly(butadiene-block-vinylpyridine) (PBD-b-PVP),poly(isoprene-block-methyl methacrylate) (PI-b-PMMA), andpoly(styrene-block-dymethylsiloxane) (PS-b-PDMS).

The specific self-assembled periodic patterns formed by the blockcopolymer are determined by the molecular volume ratio between the firstand second polymeric block components A and B. When the ratio of themolecular volume of the second polymeric block component B over themolecular volume of the first polymeric block component A is less thanabout 80:20 but greater than about 60:40, the block copolymer will forman ordered array of cylinders composed of the first polymeric blockcomponent A in a matrix composed of the second polymeric block componentB. When the ratio of the molecular volume of the first polymeric blockcomponent A over the molecular volume of the second polymeric blockcomponent B is less than about 60:40 but is greater than about 40:60,the block copolymer will form alternating lamellae composed of the firstand second polymeric block components A and B. In the present invention,the un-removed component is to be used as an etch mask for forming thegenerally radial lines, as shown in FIG. 5B, so ordered arrays ofalternating lamellae and alternating cylinders are of interest.

The periodicity or bulk period (L₀) of the repeating structural units inthe periodic pattern is determined by intrinsic polymeric propertiessuch as the degree of polymerization N and the Flory-Huggins interactionparameter χ. L₀ scales with the degree of polymerization N, which inturn correlates with the molecular weight M. Therefore, by adjusting thetotal molecular weight of the block copolymer of the present invention,the bulk period (L₀) of the repeating structural units can be selected.

To form the self-assembled periodic patterns, the block copolymer isfirst dissolved in a suitable solvent system to form a block copolymersolution, which is then applied onto the substrate surface to form athin block copolymer layer, followed by annealing of the thin blockcopolymer layer, which causes phase separation between the differentpolymeric block components contained in the block copolymer. The solventsystem used for dissolving the block copolymer and forming the blockcopolymer solution may comprise any suitable solvent, including, but notlimited to: toluene, propylene glycol monomethyl ether acetate (PGMEA),propylene glycol monomethyl ether (PGME), and acetone. The blockcopolymer solution can be applied to the substrate surface by anysuitable techniques, including, but not limited to: spin casting,coating, spraying, ink coating, dip coating, etc. Preferably, the blockcopolymer solution is spin cast onto the substrate surface to form athin block copolymer layer. After application of the thin blockcopolymer layer onto the substrate surface, the entire substrate isannealed to effectuate microphase segregation of the different blockcomponents contained by the block copolymer, thereby forming theperiodic patterns with repeating structural units.

The block copolymer films in the above-described techniquesself-assemble without any direction or guidance. This undirectedself-assembly results in patterns with defects so it is not practicalfor applications that require long-range ordering, such as for makingannular bands of radial lines on a master mold for nanoimprintingpatterned-media disks.

Lithographically patterned surfaces have been proposed to guide ordirect the self-assembly of block copolymer domains. One approach usesinterferometric lithography to achieve ordering of the domains withregistration of the underlying chemical contrast pattern on thesubstrate. Lamellar and cylindrical domains may be formed on a substrateby this technique, as described in U.S. Pat. No. 6,746,825. However,interferometric lithography cannot be used to make annular bands ofradial lines. US 2006/0134556 A1 describes techniques for creating achemical contrast pattern to guide the self-assembly of block copolymersto form aperiodic patterns. Also, in both of these approaches to createchemical contrast patterns on the substrate to guide the self-assemblyof block copolymers, the periodicity of the underlying chemical contrastpattern matches the bulk period L₀ of the block copolymer. For example,in US 2006/0134556 A1, L₀ is about 40 nm, so thelithographically-patterned substrate used to guide the self-assemblyalso has a period of about 40 nm, which can be achieved by conventionalor e-beam lithography. However, it is difficult to use conventional ore-beam lithography to create a chemical contrast pattern for a blockcopolymer with L₀ between about 8 nm and 25 nm.

A first embodiment of the method of this invention for making the mastermold will now be explained with respect to FIGS. 6A-6L. FIGS. 6A-6C,6E-6G and 6I are side sectional views, at various stages of thefabrication method, taken through a plane generally perpendicular to theradial direction, and FIGS. 6D, 6H and 6J-6L are top views at variousstages of the method.

In this first embodiment of the method, as shown in FIG. 6A, the mastermold substrate comprises a base 200, which may be formed of Si or SiO₂,a first substrate layer 202, which is preferably about a 10 nm thickamorphous carbon layer, and second substrate layer 204, which ispreferably about a 5 nm thick layer of germanium (Ge). Othercombinations of materials can be used, as long as materials and etchantscan be chosen to allow selective removal of the materials (withoutdisturbing the others) as needed for the described processes thatfollow. Examples of materials for the first layer 202 include Al,carbon, Cr, Si₃N₄, and variety of other materials that can withstand areactive-ion-etch (RIE) process used to etch the quartz substrate. If adifferent substrate material is used, the choice of materials for layer202 is further broadened. For the second layer 204, one of the followingmaterials (but different from the material of the first layer 202) maybe used: Cr, Al, SiO₂, Si, Ge, carbon, Si₃N₄, W, or a variety of othermaterials, as long as they can withstand the RIE used to etch the firstlayer.

The Ge layer 204 will have a native Ge-oxide film on its surface. Aneutral layer 205 of a material that does not show a strong wettingaffinity by one of the polymer blocks over the other is deposited ontothe Ge layer 204. The neutral layer can be, but is not restricted to, afunctionalized polymer brush, a cross-linkable polymer, a functionalizedpolymer “A” or “B” or a functionalized random copolymer “A-r-B” or ablend of “A” and “B”, where “A” and “B” are the constituent blockmaterials of the block copolymer. The functional group may be, forexample, a hydroxyl group. In the present example, the neutral layer 205is a hydroxyl-terminated polystyrene brush of lower molecular weightthan the block copolymer used. The brush material is spin-coated on Gelayer 204 to a thickness of about 1-10 nm (below 6 nm is preferred). Thepurpose of the neutral layer is to tune the surface energy adequately topromote the desired domain orientation (perpendicular lamellae orparallel cylinders) and to provide the adequate wetting conditions fordensity multiplication.

In FIG. 6B a resist layer has been deposited on brush layer 205 andpatterned into generally radial bars 210 of resist. The resist layer ispatterned by e-beam and developed to form the pattern of radial bars 210separated by radial spaces 211 that expose portions of brush layer 205.The e-beam tool patterns the resist layer so that the radial spaces 211have a circumferential spacing that is approximately an integer multipleof L₀ (i.e., nL₀), the known bulk period for the selected blockcopolymer that will be subsequently deposited. In FIG. 6B, n is 2. Thecircumferential width of each radial space 211 is selected to beapproximately 0.5 L₀.

In FIG. 6C, the structure is etched, by a process of oxygen plasmareactive ion etching (O₂ RIE), to remove portions of brush layer 205 inthe radial spaces 211, which exposes portions of Ge layer as generallyradial stripes 204. Alternatively, the chemical structure of the exposedportions of brush layer 205 in the radial spaces 211 can be altered sothat they have a preferred affinity for one of the copolymers. In FIG.6D, which is a top view, the resist 210 is removed, leaving on thesubstrate a pattern of generally radial bars 205 of polymer brushmaterial separated by generally radial stripes 204 of Ge. In thispattern the generally radial stripes 204 have a circumferential width of0.5 L₀ and a circumferential pitch of 2 L₀. The structure in FIG. 6D,with generally radial stripes 204, corresponds generally to the stepshown in FIG. 5A. Because FIG. 6D is only a very small portion of themaster mold, the stripes 204 appear as parallel stripes. However, thestripes 204 are arranged generally radially, as depicted in FIG. 4. Thestripes 204 may be perfectly straight radial stripes but are preferablyarcs or arcuate-shaped radial stripes that replicate the arcuate path ofthe read/write head on the rotary actuator.

Next, in FIG. 6E, a layer 220 of block copolymer material is depositedover the radial bars 205 of brush material and the radial stripes 204 ofGe layer (or chemically altered brush) in the radial spaces 211. Thepreferred block copolymer material is the diblock copolymerpolystyrene-block-polymethylmethacrylate (PS-b-PMMA) with L₀ betweenabout 8 nm and 25 nm and is deposited by spin coating to a thickness ofabout 0.5 L₀ to 3 L₀.

In FIG. 6F, the block copolymer layer has been annealed, which resultsin phase separation between the different components contained in theblock copolymer. In this example, the B component (PMMA) has an affinityfor the oxide surface of the Ge stripes or for the polar groups of thechemically altered brush 204 and thus form as generally radial lines 215on top of the radial Ge stripes 204. Because the circumferential widthof the Ge stripes 204 is approximately 0.5 L₀, the A component (PS) formin adjacent radial lines 212 on the radial bars 205 of polymer brushmaterial. As a result of the self-assembly of the A and B componentsthis causes the B component to also form as generally radial lines 215on the centers of each radial bar 205 of polymer brush material. Thegenerally radial Ge stripes 204 (or chemically altered brush) thus guidethe self-assembly of the PS and PMMA components to form the alternatingradial lines 212, 215 in the structure as shown in FIG. 6F. Although theA and B components prefer to self-assemble in parallel lines with aperiod of L₀, the substrate pattern of radial stripes 204 guides thealternating lines 212, 215 to form as radial lines, which means thatthat L₀ cannot be constant over the entire radial length. However, apattern of alternating radial lines 212, 215 can be accomplished withoutany significant defects if the variation from L₀ does not exceedapproximately 10 percent. Thus, to achieve this, the circumferentialspacing of the radial stripes 204 at the band ID should not be less thanabout 0.9nL₀ and the circumferential spacing of the radial stripes 204at the band OD should not be greater than about 1.1nL₀.

Next, in FIG. 6G, the B component (PMMA) is selectively removed by a wetetch (acetic acid, IPA or other selective solvent) or a dry etch process(O₂ RIE), leaving generally radial lines 212 of the A component (PS).FIG. 6H is a top view of FIG. 6G and shows the generally radialA-component lines 212 with a circumferential spacing L₀. FIG. 6Hcorresponds generally to the step shown in FIG. 5B, where thecircumferential density of radial lines 212 has been doubled from thecircumferential density of radial stripes 204 in FIG. 5A.

In FIG. 6I, the Ge layer 204 has been etched, using the PS radial lines212 as an etch mask, by a fluorine-based reactive-ion-etch (RIE)process. The PS material in radial lines 212 and the underlying polymerbrush layer 205 has been removed by a dry etch process (O₂ RIE). Thisleaves generally radially lines 208 of Ge on carbon layer 202. The Gelines 208 have the same circumferential spacing L₀ as the radial lines212 of PS material in FIG. 6H.

Next, a second conventional e-beam or other lithography step isperformed to cut the radial lines 208 into segments that will correspondto the tracks on the patterned-media disks that will be nanoimprinted bythe master mold. In FIG. 6J, which is a top view, the structure of FIG.6I is coated with a layer of e-beam resist 217. Then the resist 217 isexposed in a rotary-stage e-beam tool to expose narrow concentricboundary regions 207 that correspond to the boundaries between thetracks of the patterned-media disks to be nanoimprinted. The resist 217may be a positive e-beam resist like poly methyl methacrylate (PMMA) orZEP520 from Zeon Chemicals, L.P. After developing, this will leavecircumferential segments 213, which correspond to the tracks on thepatterned-media disks to be nanoimprinted, covered with resist 217, withthe boundary regions 207 between tracks not covered with resist. Byadjusting the exposure and developing conditions, the width of theuncovered boundary regions can be adjusted as desired.

In FIG. 6K, a fluorine-based RIE is used to etch the exposed Ge in theboundary regions 207. Then the resist 217 is removed in a wet etchprocess, like hot N-methyl pyrrolidone (NMP), or a dry etch process,like oxygen RIE. This leaves Ge pillars 226 on carbon layer 202.

Then, in FIG. 6L, an oxygen RIE is used to etch the carbon layer 202from the substrate 200 in the regions between the Ge pillars 226, usingthe Ge pillars 226 as an etch mask. The Ge pillars 226 are then removedby a fluorine RIE process, leaving carbon pillars 228 on substrate base200. This leaves the structure as shown in FIG. 6L with the carbonpillars 228 being arranged in circumferential segments 213 whichcorrespond to the concentric tracks of the patterned-media disks to benanoimprinted. The resulting structure in FIG. 6M corresponds generallyto the step shown in FIG. 5C. The carbon pillars 228 have a IAR ofgreater than 1, preferably about 2 or greater. The structure of FIG. 6L,which began as a substrate of base 200 with carbon layer 202 and Gelayer 204, has now been etched so that a portion of the substrateremains as the topographic pattern in the form of carbon pillars 228.The structure of FIG. 6L can function as the master mold with the carbonpillars 228 functioning as the topographic pattern for nanoimprintingthe replica molds. As an alternative approach, the carbon pillars 228 inFIG. 6L can function as an etch mask for an additional etching step toetch the underlying substrate base 200 using a fluorine RIE process.After etching and removal of the carbon pillars 228 in this alternativeapproach, the result would be a master mold wherein the pillars areformed of the same material as the substrate base 200.

In FIG. 6L, the pillars 228 have a circumferential pitch of about 15 nmand a circumferential width of about 8 nm and a radial length of about25 nm with a radial pitch of about 30 nm, resulting in a BAR of greaterthan about 2. The 15 nm pillar spacing or pitch in the circumferentialdirection corresponds approximately to L₀ and is half of that used inthe e-beam lithography step which defined the radial stripes 204 in FIG.6D. This array may be used as a master mold for nanoimprintingpatterned-media disks with a density of about 1.4 Gigabit/in².

An alternative to the steps shown in FIGS. 6C-6F will also generate thenecessary pattern on the substrate. This is shown in FIGS. 6M-6Q. Thestructure in FIG. 6B is used as the starting point. In FIG. 6M amaterial 250 with a wetting affinity for one of the two polymer blocks,such as SiO₂, is deposited in the radial spaces 211 to a thickness ofabout 1 nm. In FIG. 6N, the radial bars 210 of resist are removed,leaving on the substrate a pattern of generally radial stripes 250 ofSiO₂ separated by generally radial spaces 205 of polymer brush material.In this pattern the generally radial stripes 250 have a circumferentialwidth of 0.5 L₀ and a circumferential pitch of 2 L₀. Because FIG. 6N isonly a very small portion of the master mold, the stripes 250 appear asparallel stripes. However, the stripes 250 are arranged generallyradially, as depicted in FIG. 4. The stripes 250 may be perfectlystraight radial stripes but are preferably arcs or arcuate-shaped radialstripes that replicate the arcuate path of the read/write head on therotary actuator. Next, in FIG. 6O, a layer of block copolymer materialis deposited over the radial stripes 250 of SiO₂ and the radial spaces205 of polymer brush material. The preferred block copolymer material isthe diblock copolymer polystyrene-block-polymethylmethacrylate(PS-b-PMMA) with L₀ between about 8 nm and 25 nm and is deposited byspin coating to a thickness of about 0.5 L₀ to 3 L₀. In FIG. 6O, theblock copolymer layer has been annealed, which results in phaseseparation between the different components contained in the blockcopolymer. In this example, the B component (PMMA) has an affinity forthe oxide surface of the radial SiO₂ stripes 250 and thus form asgenerally radial lines 215 on top of the radial SiO₂ stripes 250.Because the circumferential width of the radial SiO₂ stripes 250 isapproximately 0.5 L₀, the A component (PS) form in adjacent radial lines212 in the radial spaces 205 of polymer brush material. As a result ofthe self-assembly of the A and B components this causes the B componentto also form as generally radial lines 215 in the centers of each radialspace 205 of polymer brush material. The generally radial SiO₂ stripes250 thus guide the self-assembly of the PS and PMMA components to formthe alternating radial lines 212, 215 in the structure as shown in FIG.6O. Next, in FIG. 6P, the B component (PMMA) is selectively removed by awet etch (acetic acid, iso-propyl alcohol (IPA) or other selectivesolvent) or dry etch process (O₂ RIE), leaving generally radial lines212 of the A component (PS) and the generally radial SiO₂ stripes 250.In FIG. 6Q the radial SiO₂ stripes 250 have been removed by a selectivewet or dry etching. For SiO₂ this would be hydrofluoric acid for the wetetching or a CHF₄ plasma for a dry etch. The remaining exposed brushlayer 205 in the spaces between radial lines 212 is then removed by O₂RIE, leaving generally radial lines 212 of the A component (PS) and thegenerally radial lines 204 of Ge. At this point the structure isidentical to that shown in FIG. 6G and the process can continue asdescribed above.

A variation of the embodiment of the methods shown in FIGS. 6A-6O isshown in FIGS. 7A-7E, wherein FIG. 7A is a side sectional view takenthrough a plane generally perpendicular to the radial direction, andFIGS. 7B-7E are top views at various stages of the method. In thisvariation, as shown in FIG. 7A, the master mold substrate is base 200which is a Si substrate with a native oxide layer. FIG. 7A correspondsto the stage of the method shown by FIG. 6G. The B component (PMMA) hasbeen dissolved by a wet etch or dry etch process, leaving generallyradial lines 212 of the A component (PS). FIG. 7B is a top view of FIG.7A and shows the generally radial lines 212 with a circumferentialspacing L₀. FIG. 7A corresponds generally to the step shown in FIG. 5B,where the circumferential density of radial lines 212 has been doubledfrom the circumferential density of radial stripes 204 in FIG. 5A.

Next, a second conventional e-beam or other lithography step isperformed to cut the radial lines 212 into segments that will correspondto the tracks on the patterned-media disks that will be nanoimprinted bythe master mold. In FIG. 7C, which is a top view, the structure of FIGS.7A-7B is coated with a layer of e-beam resist 217. Then the resist 217is exposed in a rotary-stage e-beam tool to expose narrow concentricboundary regions 207 that correspond to the boundaries between thetracks of the patterned-media disks to be nanoimprinted. The resist 217may be a positive e-beam resist like PMMA or ZEP520. After developing,this will leave circumferential segments 213, which correspond to thetracks on the patterned-media disks to be nanoimprinted, covered withresist 217, with the boundary regions 207 between tracks not coveredwith resist. By adjusting the exposure and developing conditions, thewidth of the uncovered boundary regions can be adjusted as desired.

Next, in FIG. 7D, the PS (block copolymer component A) in the exposedportions of radial lines 212 in the boundary regions 207 is removed by aO₂ RIE process. Then the resist 217 is removed in a wet etch process,like hot NMP. This leaves pillars 226′ of PS on substrate 200.

Then, in FIG. 7E, a dry etch process is used to etch the substrate 200in the regions between the PS pillars 226′, using the PS pillars 226′ asan etch mask. The PS pillars 226′ are then removed by a O₂ RIE process,leaving pillars 228′ of substrate material on substrate 200. This leavesthe structure as shown in FIG. 7E with the pillars 228′ being arrangedin circumferential segments 213 which correspond to the concentrictracks of the patterned-media disks to be nanoimprinted. The resultingstructure in FIG. 7E corresponds generally to the step shown in FIG. 5C.The structure of FIG. 7E, which began as a substrate of base 200, hasnow been etched so that a portion of the substrate remains as thetopographic pattern in the form of pillars 218′. The structure of FIG.7E can function as the master mold with the pillars 218′ functioning asthe topographic pattern for nanoimprinting the replica molds.

Another embodiment of the method for making the master mold is shown inFIGS. 8A-C. The structure shown in FIG. 8A, which is a side sectionalview taken through a plane generally perpendicular to the radialdirection, is made by starting with a substrate 300, which may be formedof Si or SiO₂. Then a layer 305 of neutral polymer brush material isdeposited over the entire surface of substrate 300. A layer of resist isthen deposited over brush layer 305 and patterned with conventionaloptical or e-beam lithography. After exposure and development a patternof generally radial stripes are formed that expose the underlying brushlayer 305. The brush material is removed from these regions by a O₂ RIEprocess and then a material like Ge or amorphous Si is deposited overthe patterned resist into the regions of radial stripes. Then the resistis removed, leaving the structure shown in FIG. 8A with generally radialstripes 304 that define a trench with a circumferential width nL₀, wheren is an integer greater than or equal to 2 (n=9 in the example of FIG.8A). The bottom of the trench is formed of the neutral brush layer 305and the walls 304 a of radial stripes 304 have a native oxide surface.

Next, in FIG. 8B, a layer of block copolymer material has been depositedbetween the radial stripes 304 and onto neutral brush layer 305. Thepreferred block copolymer material is the diblock copolymerpolystyrene-block-polymethylmethacrylate (PS-b-PMMA) with L₀ betweenabout 8 nm and 25 nm and is deposited by spin coating to a thickness ofabout 0.5 L₀ to 3 L₀. The block copolymer is then annealed, whichresults in phase separation between the different components containedin the block copolymer. In this example, the B component (PMMA) has anaffinity for the oxide surfaces of the walls 304 a of radial stripes304. Because the B component is formed on the walls 304 a of the radialstripes 304 as radial lines 215 a and because the circumferential widthbetween the walls 304 a of radial stripes 304 is an integer multiple ofL₀ (9 L₀ in the example of FIG. 8B), the A and B components betweenradial B-component lines 215 a are forced to self-assemble inalternating radial lines 212, 215, each with a circumferential width ofapproximately 0.5 L₀. The circumferential width of B-component radiallines 215 a, however, is approximately 0.25 L₀. The generally radialstripes 304, which are spaced apart to define a trench withcircumferential width of approximately nL₀, thus guide the self-assemblyof the PS and PMMA components to form the alternating radial lines 212,215 in the structure as shown in FIG. 8B. Although the A and Bcomponents prefer to self-assemble in parallel lines with a period ofL₀, the pattern of radial stripes 304 on the substrate 300 guides thealternating lines 212, 215 to form as radial lines, which means that L₀cannot be constant over the entire radial length of the band. However,the pattern of alternating radial lines 212, 215 can be accomplishedwithout any significant defects if the variation from L₀ does not exceedapproximately 10 percent. Thus the circumferential width of the trenchat the band ID should not be less than about 0.9 nL₀ and thecircumferential width of the trench at the band OD should not be greaterthan about 1.1nL₀.

Next, in FIG. 8C, the B component (PMMA), which formed the radial lines215 a and 215 in FIG. 8B, have been dissolved by a wet etch process(acetic acid, IPA or other selective solvent) or by a dry etch process(O₂ RIE), leaving generally radial lines 212 of the A component (PS).The structure of FIG. 8C is thus at the stage corresponding to the stageshown in FIG. 7A of the first embodiment. Thus, following the stage ofFIG. 8C, the same steps as described above with respect to FIGS. 7B-7Eare followed to arrive at a master mold substantially identical to themaster mold shown in FIG. 7E. However, patterned-media disksnanoimprinted by the master mold made by the method shown in FIGS. 8A-8Cwill have missing bits at predictable regular intervals in each track asa result of the radials stripes 304 which are needed to define thecircumferential trenches. In patterned-media disks, a relativelyconstant bit spacing is desired for accurate write synchronization andreadback detection because if the spacing between the bits is notconstant, phase errors may occur. If the phase error due to the missingbits becomes more than a few percent of the bit spacing, the errors maybe unacceptable. A modification to the method of FIGS. 8A-8C will avoidany phase errors due to the radial stripes 304. This can be achieved byplacing the stripes 304 such that the walls 304 a are spaced apart by awidth of (n+½)L₀, for example 9.5 L₀ in FIG. 8A. The circumferentialwidth of the radial stripes 304 is selected to be 0.5 L₀. Each wallsurface 304 a is then coated with a PMMA brush of thickness about 0.25L₀, so that the spacing between the coated walls is 9 L₀. Then the blockcopolymer is applied and the process is the same as above. The radialstripes 304 substitute for one PS stripe while guiding the self-assemblyof the block copolymer. The PMMA brush on the walls 304 a compensate forthe missing material.

In the embodiment of the method described with respect to FIGS. 6A-6L(and the variation described with respect to FIGS. 7A-7E) and in theembodiment of the method described with respect to FIGS. 8A-8C, the twoblock copolymer components are depicted as self-assembling intoalternating lamellae, as shown, for example, by alternating radial lines212, 215 in FIG. 6F. For the A and B components (PS and PMMA) to form asalternating lamellae the molecular weight ratio of the A to B componentsshould be between about 40:60 and 60:40, preferably close to 50:50.However, it is also within the scope of the invention for the Acomponent (PS) to form as radially-aligned cylinders within a matrix ofthe B component (PMMA). To achieve this type of structure, wherein the Acomponent cylinders form the radial lines 212 within alternating radiallines 215 of B component material, the molecular weight ratio ofcomponent B over component A should be less than about 80:20 but greaterthan about 60:40, preferably close to 70:30.

The master mold shown in FIGS. 6L and 7E is a pillar-type master moldthat can be used to make replica molds. The replica molds will thus havehole patterns corresponding to the pillar pattern of the master mold.When the replica mold is used to make the disks, the resulting diskswill then have a pillar pattern, with the pillars corresponding to thedata islands. However, the master mold may alternatively be a hole-typeof master mold that can be used to directly nanoimprint the disks.

While the present invention has been particularly shown and describedwith reference to the preferred embodiments, it will be understood bythose skilled in the art that various changes in form and detail may bemade without departing from the spirit and scope of the invention.Accordingly, the disclosed invention is to be considered merely asillustrative and limited in scope only as specified in the appendedclaims.

1. A method for making a master mold for use in imprinting magneticrecording disks, the method including the use of a block copolymerhaving a bulk period L₀ and comprising: forming on a substrate having acenter a pattern of generally radial stripes arranged in an annularband, the stripes having a circumferential spacing of approximately nL₀,where n is an integer equal to or greater than 2; and depositing on thepatterned substrate a layer of material comprising a block copolymerhaving a bulk period L₀, whereby the copolymer material is guided by thestripes to self-assemble into generally radial lines of alternatingfirst and second copolymer components, the radial lines of eachcomponent having a circumferential spacing of approximately L₀.
 2. Themethod of claim 1 wherein the copolymer material is a diblock copolymermaterial.
 3. The method of claim 2 wherein the diblock copolymermaterial is a copolymer of polystyrene (PS) and poly(methylmethacrylate) (PMMA).
 4. The method of claim 1 wherein L₀ is betweenabout 8 nm and 25 nm.
 5. The method of claim 1 wherein thecircumferential spacing of the generally radial stripes at the insidediameter (ID) of the annular band is not less than about 0.9nL₀ and thecircumferential spacing of the generally radial stripes at the outsidediameter (OD) of the annular band is not greater than about 1.1nL₀. 6.The method of claim 1 wherein the generally radial stripes have agenerally arcuate shape.
 7. The method of claim 1 wherein forming on thesubstrate a pattern of generally radial stripes arranged in an annularband comprises forming the stripes as a plurality of radially-spacedannular bands.
 8. The method of claim 1 wherein the substrate surfacecomprises an oxide and wherein forming said pattern on the substratecomprises: depositing on the substrate a layer of neutral polymer brushmaterial having no strong affinity for any copolymer component; formingon the polymer brush layer a resist pattern of generally radial bars;etching portions of the polymer brush material unprotected by the resistto expose generally radial stripes of oxide having said circumferentialspacing of approximately nL₀; and removing the resist.
 9. The method ofclaim 1 wherein forming said pattern on the substrate comprises:depositing on the substrate a layer of neutral polymer brush materialhaving no strong affinity for any copolymer component; forming on thepolymer brush layer a resist pattern of generally radial bars; formingin the radial spaces between the radial bars a material having apreferred affinity for one of the copolymer components; removing theradial bars of resist to expose generally radial stripes of saidmaterial having a preferred affinity for one of the copolymercomponents, the radial stripes having said circumferential spacing ofapproximately nL₀.
 10. The method of claim 1 wherein forming saidpattern on the substrate comprises forming generally radial ridges, theridges defining trenches with a circumferential width of approximatelynL₀, and further comprising, prior to depositing the layer of blockcopolymer material, depositing on the substrate in the trenches betweenthe radial ridges a layer of neutral polymer brush material having nopreferred affinity for any copolymer component.
 11. The method of claim10 where the ridges have walls, the walls of circumferentially adjacentridges being spaced apart by width about (n+0.5)L₀, the walls beingcoated with polymer brush material having a thickness of about 0.25 L₀.12. The method of claim 1 further comprising: removing the radial linesof the first copolymer component, leaving the radial lines of the secondcopolymer component; forming over the radial lines of the secondcopolymer component a resist pattern of concentric rings; etchingportions of the radial lines of the second copolymer componentunprotected by the resist; removing the resist, leaving a pattern ofpillars of second copolymer component; etching the substrate, using thepillars of second copolymer component as an etch mask; and removing thepillars of second copolymer component, leaving a substrate with apattern of pillars of substrate material.
 13. The method of claim 12wherein the substrate comprises a substrate base having a substratelayer formed on the base, wherein etching the substrate comprisesetching the substrate layer, and wherein after removing the pillars ofsecond copolymer component, a pattern of pillars of substrate layermaterial is left on the base.
 14. The method of claim 12 wherein theratio of radial spacing of the concentric rings of substrate materialpillars to the circumferential spacing of the substrate material pillarsis greater than
 1. 15. The method of claim 1 wherein the substratecomprises a substrate base having a substrate layer formed on the baseand further comprising: removing the generally radial lines of the firstcopolymer component, leaving exposed generally radial lines of substratelayer material between the radial lines of the second copolymercomponent; etching the exposed radial lines of substrate layer materialusing the radial lines of the second copolymer component as an etchmask; removing the radial lines of the second copolymer component,leaving radial lines of underlying substrate layer material; formingover the radial lines of the substrate layer material a resist patternof concentric rings; etching portions of the radial lines of substratelayer material unprotected by the resist; and removing the resist,leaving a pattern of pillars of substrate layer material on thesubstrate base.
 16. The method of claim 15 further comprising etchingthe substrate base, using the pillars of substrate layer material as anetch mask.
 17. The method of claim 15 wherein the substrate layercomprises germanium (Ge).
 18. The method of claim 15 wherein the ratioof radial spacing of the concentric rings of substrate layer materialpillars to the circumferential spacing of the substrate layer materialpillars is greater than
 1. 19. A method for making a master mold for usein imprinting magnetic recording disks, the method including the use ofa block copolymer having A and B components and a bulk period L₀ andcomprising: providing a substrate having an oxide surface; depositing onthe oxide surface of the substrate a layer of neutral polymer brushmaterial having no preferred affinity for any copolymer component;forming on the polymer brush layer a resist pattern of generally radialbars having a circumferential spacing of approximately nL₀, where n isan integer equal to or greater than 2; etching portions of the polymerbrush material unprotected by the resist to expose generally radialstripes of oxide, the radial oxide stripes having said circumferentialspacing of approximately nL₀; removing the resist, leaving generallyradial bars of polymer brush material; depositing on the radial oxidestripes and radial bars of polymer brush material a layer of blockcopolymer material having A and B components and a bulk period L₀,whereby the copolymer material is guided by the radial oxide stripes toself-assemble into generally radial lines of alternating A and Bcomponents, the radial lines of each component having a circumferentialspacing of approximately L₀.
 20. The method of claim 19 wherein thecopolymer material is a diblock copolymer material of polystyrene (PS) Acomponent and poly(methyl methacrylate) (PMMA) B component.
 21. Themethod of claim 19 wherein L₀ is between about 8 nm and 25 nm.
 22. Themethod of claim 19 wherein the generally radial oxide stripes have agenerally arcuate shape.
 23. The method of claim 19 wherein forming aresist pattern of generally radial bars comprises forming the generallyradial bars in a plurality of radially-spaced annular bands, whereby thegenerally radial oxide stripes are arranged in a plurality ofradially-spaced annular bands.
 24. The method of claim 23 wherein thecircumferential spacing of the generally radial oxide stripes at theinside diameter (ID) of each annular band is not less than about 0.9nL₀and the circumferential spacing of the generally radial oxide stripes atthe outside diameter (OD) of each annular band is not greater than about1.1nL₀.
 25. The method of claim 19 wherein the substrate comprises asubstrate base having a substrate layer formed on the base, thesubstrate layer having an oxide surface, and further comprising:removing the generally radial lines of the B component, leaving exposedgenerally radial lines of substrate layer material between the radiallines of the A component; etching the exposed radial lines of substratelayer material using the radial lines of the A component as an etchmask; removing the radial lines of the A component, leaving radial linesof underlying substrate layer material; forming over the radial lines ofthe substrate layer material a resist pattern of concentric rings;etching portions of the radial lines of substrate layer materialunprotected by the resist; and removing the resist, leaving a pattern ofpillars of substrate layer material on the substrate base.
 26. Themethod of claim 25 further comprising etching the substrate base, usingthe pillars of substrate layer material as an etch mask.
 27. The methodof claim 25 wherein the substrate layer comprises germanium (Ge),wherein the substrate further comprises a layer of carbon between thesubstrate base and the Ge layer, and further comprising: etching thecarbon layer using Ge pillars as an etch mask and thereafter removingthe Ge pillars, leaving carbon pillars on the substrate base; andetching the substrate base using the carbon pillars as an etch mask. 28.The method of claim 25 wherein the ratio of radial spacing of theconcentric rings of substrate layer material pillars to thecircumferential spacing of the substrate layer material pillars isgreater than 1.